Over the past few years considerable efforts have been expended in developing a family of zinc-aluminum alloys for structural purposes. Zinc-aluminum alloys find wide engineering application and may be attractive alternatives to cast iron, steel, and copper-base alloys. Interest has focused in recent years on hypereutectic alloys containing 8%, 11%, 27% and 35% aluminum respectively and designated ZA8, ZA11, ZA27 and ZA35. The zinc-aluminum eutectic composition is about Zn 5.1% Al. All of these alloys are relatively high strength ductile alloys which can be used in wrought condition. Those alloys close to the eutectic composition can also be used in the cast condition in a relatively heavy section but it has been found that, as the percentage of aluminum increases, a problem known as "underside shrinkage" increases. That is to say, instead of the normal shrinkage pipe at the top of the ingot or casting, a shrinkage cavity appears at the bottom of the ingot or casting. Normally shrinkage pipes are controlled by the use of hot tops or additions of hot metal, but of course these techniques are of no value in controlling underside shrinkage. Some alleviation of the problem, if it is not too severe, can be obtained by the use of chilled moulds but in the higher aluminum alloys even this technique is insufficient to remove the underside shrinkage cavity and this defect does, of course, limit the size of section which can be soundly cast. Prior attempts to solve the shrinkage problem by modifying the alloy composition by including such additions as 0.02-0.2 wt% of Groups IIa or Ia alkaline earth metals such as barium, calcium and strontium or lithium, sodium and potassium have not been particularly successful as the results of such additions have not been found to be always reproducible (M. Sahoo, L. V. Whiting and D. W. G. White, "Control of Underside Shrinkage in Zinc-Aluminum Foundry Alloys by the Addition of Trace Elements", Minerals Research Program, PMRL Division Report MRP/PMRL 84-86 (OP-J)).
Low aluminum-zinc alloys (generally recognized to contain from about 3% to about 15% aluminum) containing additions from about 5 ppm to about 1% of a rare earth containing alloy, for use as hot-dip galvanizing alloys are known (see, for example, U.S. Pat. No. 4,448,748 issued May 15, 1984 to Radtke et al. and assigned to the International Lead Zinc Research Organization Inc.) but there has been no suggestion that such alloys may be cast without underside shrinkage.
Grain refining amounts of misch metal of the order of 0.05 wt% in Zn 27 Al alloys have also been described (Metall. 37 Jahrgang. Heft 9. September 1983), but here again the amounts of misch metal added is insufficient to remedy the problem of underside shrinkage. Grain refining amounts of Ce or La between 5 ppm and 0.1 wt% are known to improve ductility of Zn 27 Al alloys without adversely affecting the tensile strength (U.S. Pat. No. 4,609,529 issued Sept. 2, 1986 to Skenazi et al.).